Parallel computer

ABSTRACT

A massively parallel computer system employs hundreds or thousands of otherwise-idle microprocessors in set-top boxes distributed throughout a cable television system. Coupling between these processing nodes is effected by the cable television distribution network, which provides an asymmetrical transmission speed characteristic that is well suited for many parallel applications. The system is tolerant of interruptions due to preemption of set-top boxes for television viewing.

RELATED APPLICATION DATA

This application is related to Ser. No. 8/539,417, filed on even date herewith.

FIELD OF THE INVENTION

The present invention relates to large scale computers employing parallel architectures.

BACKGROUND AND SUMMARY OF THE INVENTION

Computers with parallel architectures (e.g. several processing nodes working on different subparts of a computational task) are well suited for a variety of different applications. For expository convenience, the present specification focuses on one such application: computers for performing MPEG compression. It will be recognized, however, that the invention can be advantageously employed in many other applications requiring large scale parallelism.

The information superhighway holds great promise for the distribution of entertainment programming, as well as information. A problem, however, is the massive data size of entertainment programming, such as broadcast-quality video/audio. A single second of video programming includes about 54 megabytes of data (1.8 MB frame, 30 frames per second). A two hour movie thus represents about 389 gigabytes of data. The technical and logistical challenges associated with storing and transmitting this volume of data are enormous.

To address this problem, various video/audio compression techniques are known. One of these, MPEG II (Motion Pictures Expert Group), is described by ISO/IEC Standard 13818 and promises to provide a several hundred-fold reduction in data size while maintaining broadcast quality. Compression by a factor of 200 reduces the storage requirements for a two hour movie to less than two gigabytes, and reduces the requisite transmission speed down to 270 kilobytes per second. When this data is received at a subscriber's premises, it can be expanded back to substantially its original form by an MPEG decompressor, e.g. built into a set-top cable box.

The only difficulty that remains is to compress all of the entertainment programming ever produced so that it can be stored and distributed in compressed form. This is not a small undertaking.

MPEG compression is a computationally intensive process. It is so computationally intensive that even expensive parallel processors optimized for this task (e.g. the IBM PVS system) requires dozens of hours to compress a two hour movie (i.e. many times slower than real time). Less expensive devices, often in the form of arrays of customized integrated circuits, are available to perform real time compression for low performance applications, such as video conferencing, but are not suitable for use with broadcast quality material.

While specialized hardware processors offer advantages in speed, they suffer in terms of flexibility. For example, different video passages have different characteristics making them susceptible to different optimizations during the compression process. Dedicated hardware approaches are ill-suited to effect such optimizations.

In terms of flexibility, software-based MPEG compression is preferred, since the processing techniques and parameters can more easily be adapted to the particular characteristics of the video clip being compressed. But software-based techniques are generally an order of magnitude slower than comparable hardwarebased approaches.

As a hybrid, some researchers have attempted MPEG processing on a network of computers, as illustrated by Bevinakoppa et al, "Digital Image Compression on a Network of Transputers," Transputers and Parallel Applications, TAPA-92, pp. 25-32, 1993, and Yu et al, "Software Implementation of MPEG-II Video Encoding Using Socket Programming in LAN," Proc. of the SPIE Vol. 2187, pp. 229-40, 1994. But such approaches have a variety of drawbacks, including poor use of network bandwidth, the high cost of the individual processing nodes, the unavailability of the nodes for other tasks during MPEG processing, and the delay of the MPEG processing if one of these nodes is preempted for another use.

In accordance with a preferred embodiment of the present invention, the foregoing and other drawbacks of the prior art are overcome. By forming a massively parallel, distributed system using processors in interconnected and otherwise-idle cable television set-top boxes, MPEG compression tasks are partitioned and performed economically in real- or better than real-time. Processing interruptions due to subscriber uses of their cable set-top boxes are detected and handled without unduly slowing the compression process.

The foregoing and other features and advantages of the preferred embodiment of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a massively parallel distributed computing system according to one embodiment of the invention.

FIG. 2 is a block diagram of a set-top box used in the computing system of FIG. 1.

FIG. 3 is a block diagram of the central processing facility used in the computing system of FIG. 1.

FIG. 4 is a diagram showing switching of a set-top box between two operational modes.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, a massively parallel distributed computing system 10 includes hundreds, thousands, or potentially millions of set-top boxes 12 which are coupled to a cable system head end 14 through a network 16.

As shown in FIG. 2, each set-top box 12 consists of a computer processor 18, computer memory 20, MPEG decompression hardware 22, graphics hardware 24, and a network connection 26. The illustrated set-top box does not contain any local secondary storage such as a hard disk, but local secondary storage can be added. The computer processor 18 can be any general purpose CPU, such as an Intel X86, Motorola 68000-series, or PowerPC chip, or can be a chip developed especially for the set-top environment. Two or more processors working in parallel can also be used.

The illustrated computer memory 20 is high speed random access memory (RAM). However, the computer memory is not limited to high speed RAM, but can be any other high speed computer memory, optical memory, protein memory or other local secondary storage. Experience has shown that a memory size of at least 8 MB gives good performance results. However a larger or smaller memory size can be used.

The MPEG decompression hardware 22 a self-contained chip for MPEG decompression (decoding) based on the ISO standard, and is used to decompress entertainment programming transmitted to the set-top box 12 for viewing. This decompression hardware is not utilized in the preferred embodiment of the present invention.

The graphics hardware 24 is an EGA, VGA, SVGA graphics board, but can be any circuitry adapted to display high quality video.

The network connection 26 serves to interface the set-top box to the cable system. The architecture of advanced set-top boxes is well known to artisans in this field, and so is not belabored here. See, e.g., Furht et al, "Design Issues for Interactive Television Systems," IEEE Computer Magazine, May, 1995, pp. 25-39, and the references cited therein.

As shown in FIG. 3, each set-top box 12 operates in one of two distinct states. In a second state 30, the set-top box is used to perform a subscriber function, such as viewing video programming, videotext, electronic programming guides, etc. In a first state 28, the set-top box is used by the massively parallel computing system 10 as a processing unit to complete MPEG compression subtasks. The set-top box 12 typically operates in the first state (28), and is switched to the second state (30) when a user turns on the associated television set to view subscriber information.

Returning to FIG. 1, the network 16 includes coaxial cables 32, high speed switches 34, fiber optic cables 36, and a high speed, large bandwidth switching network 38.

Within a small geographical area, such as a neighborhood or apartment building, a group 40 of set-top boxes 12 are coupled together by coaxial cable 32. This same cable couples the grouping of boxes to a local high speed switch 34.

In the preferred embodiment, the local high speed switch 34 is an Asynchronous Transfer Mode (ATM) Switch that uses an ATM protocol. An ATM switch 34 uses packet switching technology, and is capable of transporting broadband data (such as video/audio) at rates greater than 100 MB per second. However, any other high speed, large bandwidth switches can be used in place of the ATM switch 34.

In more detail, the local high speed switches 34 transport video frames via ATM packets and route the data packets using the Internet Protocol (IP). IP is the network layer of the Internet communications protocol which provides a connectionless delivery service and also provides routing of data packets to the set-top boxes. The IP network layer, over the ATM data link/physical layers, provides an appropriate high speed large bandwidth data transport service. Other routing, communications protocols, including connection or connectionless services can be used in place of the IP-over-ATM combination.

The ATM protocol is divided into three layers: the physical layer, the ATM layer, and the ATM adaptation layer. The combination of the ATM layer and the ATM adaptation layer represent the data link layer of the OSI network model (The OSI model contains the physical, data link, network, transport, session, presentation, and application layers). The physical layer encodes and decodes the data into suitable electrical/optical waveforms for transmission and reception on the communication medium used. The ATM layer performs switching and multiplexing functions. The ATM adaptation layer defines a set of service classes to fit the needs of different users requests and converts incoming user requests into ATM cells for transport.

The local high speed large bandwidth switches 34 are interconnected with other local high speed large bandwidth switches by fiber optic cable 36 to form a high speed, large bandwidth switching network 38. Other high speed large bandwidth interconnections such as wireless connections, etc. can be used in place of the fiber optic cable.

The foregoing network infrastructure is illustrative only and many variants thereon, including wireless interconnections, satellite downlink/terrestrial return link, etc., can of course be used. The cited Furht paper shows a few such alternate systems.

In the illustrated embodiment, network 16 is asymmetrical in transmission capacity. Each outgoing channel (i.e. from the head end 14 to the boxes 12) transfers data at a rate of up to 10 MB/second. Each incoming channel (i.e. from the boxes back to the head end) transfers data at a rate of up to 100 KB/sec). (In the illustrated embodiment, this asymmetry is due to the hardware devices used to relay data in different directions along the network, rather than a limitation on the cables themselves.) As will be evident below, such asymmetry is well suited to an MPEG compression architecture; the compression performed at the boxes causes the incoming data rate to be on the order of one-one hundredth of the outgoing data rate.

Desirably, the network supports several (e.g. 5) 10 MB/100 KB channels of information in order to serve a large number of set-top boxes 12. In the illustrated embodiment, these different channels are transmitted in frequency multiplexed form over the network 16. Each box 12 is associated with one such channel, and data sent thereto includes addressing bits in its packet structure that uniquely identify the recipient box.

At the head-end 14 of the network is a central processing facility (CPF) 42. CPF 42 is connected to the massively parallel network by fiber optic cable 44, but other high speed large bandwidth interconnections such as wireless connections, etc. can be used.

As shown in FIG. 4, CPF 42 includes a video/audio source 46, an array of digital disk recorders (DDRs) 48, a distribution controller/frame server 50, a routing table 52, and archival storage 54.

The video/audio source 46 can be one of any number of sources, such as a broadcast quality VTR with associated analog-to-digital conversion circuitry, or a feed from a remote source (e.g. a satellite downlink), etc.

The array of DDRs 48 serve as a spooling buffer for data from the video/audio source 46, and is responsive to control signals from the distribution controller/frame server 50. In the illustrated embodiment, each DDR 48 has a capacity of about 7 GB (which corresponds to about 128 seconds of video data at 1.8 MB per frame) and can deliver the data to the distribution controller/frame server 50 at a rate in excess of 10 MB per second.

Each DDR 48 is connected to distribution controller/frame server 50 by a bus 56, such as an FW-SCSI-2 bus. The FW-SCSI-2 is an ANSI standard Input/Output (I/O) interface used primarily for attaching data storage devices. Other high bandwidth interfaces can be used in place of the FW-SCSI-2.

(Since mass storage systems of the foregoing type are well known, the details of this aspect of the system are not belabored here. The interested reader is referred to, e.g., Gemmell et al, "Multimedia Storage Servers: A Tutorial," IEEE Computer Magazine, May, 1995, pp. 40-49, and the references cited therein, for additional information.)

The distribution controller/frame server 50 is a dedicated group of computer processors that accepts uncompressed video data from the DDRs 48 and partitions the data for transmission to the set-top boxes. The distribution controller/frame server also accepts MPEG compressed video frames back from the set-top boxes and sends them to archival storage 54. Any high speed computer processor, or group of processors working in parallel, can be used for the distribution controller/frame server.

Using several 10 MB per second channels, the distribution controller/frame server 50 can provide video frame data at a rate exceeding 50 MB per second onto the massively parallel distributed network and to the set-top boxes 12 for processing. The distribution controller/frame server then receives the MPEG encoded output data back from the network at a rate below 0.5 MB per second over the several channels and provides it to the archival storage 54.

Archival storage 54 is implemented with high capacity, high speed disks. The disks in the preferred embodiment have a storage capacity of in the hundreds of giga-byte range. Large capacity archival storage other than hard disks such as optical storage media, etc. can also be used.

The distribution controller/frame server 50 maintains a routing table 52 and a data base 58 to determine how to allocate the MPEG compression subtasks. The routing table 52 contains a list of set-top boxes, their current state, and route (addressing) information. The data base 58 contains information relating to the patterns of cable television receiver usage by cable subscribers, and is used in determining subtask distribution, as discussed below.

When video data is to be compressed, CPF 42 checks the data base 58 and the routing table 52 to determine which set-top boxes are available for MPEG compression. If desired, the CPF can send a message to candidate set-top boxes to confirm that they are actually in state one (28) (FIG. 3)(i.e. not being actively used for subscriber services).

Once CPF 42 has identified boxes 12 that are available for MPEG compression, distribution controller/frame server 50 downloads a software MPEG encoder to each of the boxes that is to be used. The software MPEG encoder can follow the model of the encoder described in Appendices A and B to ISO/IEC document 13818-5, or can be any other MPEG encoder known to artisans in the field.

Next, the distribution controller/frame server begins partitioning the video stream provided from DDRs 48 for distribution to the set-top boxes. Desirably, each box is assigned a group of pictures (GOP) that may span, e.g., a half second of video.

In the preferred embodiment, the distribution controller/frame server 50 does a rudimentary analysis of each group of pictures to determine if it includes an abrupt scene transition. If so, the GOP is broken into two groups, the second beginning at the abrupt transition. By this arrangement, each group of pictures is encoded into a single "I" (initial) frame, followed by one or more "P" (motion prediction) and "B" (difference) frames. (If an abrupt scene transition is included in a single group of pictures, two "I" frames would typically be required.)

For more insight into partitioning video for MPEG compression, the reader is referred to the Yu paper cited earlier, and the references cited therein.

After partitioning, each compression subtask (i.e. GOP) is routed to one of the available set-top boxes for processing. The routing table keeps track of which box is handling which subtask.

Each box compresses the group of pictures assigned to it, and returns the corresponding compressed data back to the CPF for storage in archival storage 54. The distribution controller/frame server 50 then assigns it another group of pictures for processing.

Whenever a subscriber activates the set-top box for TV viewing, the box 12 transmits a message packet to the distribution controller/frame server 50 informing it of the change from state one (28) (not actively being used the subscriber) to state two (30) (being used by the subscriber). This data is used by the distribution controller/frame server to update the routing table 52. If this activation occurs during the box's processing of a compression subtask, the subtask is naturally interrupted. In this case, the distribution controller/frame server may send that box's group of pictures to another set-top box for compression. The new set-top box would be initialized either to continue the MPEG compression from an intermediate state, or start the MPEG compression process over from the beginning.

Desirably, reassignment of a compression subtask to a substitute set-top box is not required to handle processing interruptions. Instead, the preferred embodiment of the invention redundantly assigns each compression subtask to several different set-top boxes. By so doing, interruptions at individual boxes do not unduly delay completion of the final compression task.

If a compression subtask is assigned redundantly to several boxes, the preferred embodiment treats the subtask as completed when the first box returns its results. Results thereafter returned by the other boxes are ignored. Data integrity of the first-to-complete results is enhanced by encoding the packets containing the compressed data with error correcting codes for their return to the CPF. In other embodiments, the data returned by two or more boxes can be compared against one another to assure data integrity.

As noted, whenever a box 12 switches states (FIG. 3), it informs the CPF 42. Most processing interruptions are identified in this manner. Other interruptions, however, may also occur. For example, a consumer may unplug the set-top box, or a lineman may temporarily disconnect a residential service. Desirably, means are provided to detect such interruptions.

One such approach is for the distribution controller/frame server 50 to set an internal timer each time it assigns a subtask to a set-top box. If the set-top box doesn't return its compressed results to the CPF 42 before the timer expires, the box's state is flagged as interrupted. In another embodiment the compression software executed on each box sends out a periodic "heartbeat" to the CPF reporting the box's status. If the CPF does not receive a box's heartbeat with a predetermined interval, the box is flagged as interrupted. In either event, the system recovers just as it would if the box switched from state one (28) to state two (30).

There are times when parts of the video stream stored on DDRs 48 may require special processing (e.g. noise filtering, etc.). Any required special processing is flagged in an edit-decision-list 60 associated with the video stream at the central processing facility 42. When the video is partitioned into subtasks for individual set-top boxes, the edit-decision-list is consulted for special instructions. If a box is assigned a group of pictures for which the edit decision list has corresponding instructions, these instructions are transmitted to the box with the group of pictures. In response, the box's software encoder adjusts its encoding parameters accordingly.

Due to local memory limitations at the set-top boxes 12, the group of frames comprising a single compression subtask are not downloaded all at once to the boxes. Instead, video frames are directed to the box one at a time, with the box requesting additional frames as needed from the distribution controller/frame server 50. Likewise, compressed data frames are returned to the CPF 42 for archival storage on a frame by frame basis, as they are completed by the software encoder. (In other embodiments having more local memory at the processing nodes, other arrangements can naturally be used.)

It will be recognized that the compressed frames returned from the set-top boxes to the central processing facility 42 may not arrive in temporal order. The distribution controller/frame server 50 assembles the frames in proper order before storage in the archival storage 54. (The distribution controller/frame server 50 also handles the multiplexing of compressed audio with the compressed video to achieve the correct ordering.)

To aid in efficient distribution of processing subtasks to set-top boxes, statistical methods are used to determine subscriber television viewing habits, These statistical methods are similar to the traffic analysis done in the telecommunications industry to determine network load and routing paths for specified hours of the day.

In the preferred embodiment, subscriber viewing habits are determined using data from the Nielsen Company, which compiles ratings for the television industry. Nielsen data typically includes the time of day and duration of television viewing. This information is saved in data base 58 and is then used to determine how to spread the MPEG compression load, and what hours of the day might be best suited to achieve the best MPEG compression results. Other statistical methods or queuing theory techniques can also be used to determine subscriber television viewing habits for MPEG compression load distribution.

Since even at peak viewing times, the Nielsen ratings indicate that only about 45% of the total number television sets are actually in use, a massively parallel network consisting of as few as 5,000 set-top boxes is adequate for several real-time MPEG compressing jobs running in parallel.

To achieve real-time MPEG compression, all the video frames contained on each of the DDRs need to be processed by the time that new video data would arrive to replace that already stored on the DDRs. To compress a two hour movie, a minimum of 6 DDRs (with 7 GB capacity each) working in parallel are required. Each set-top box then has a maximum of up to 12 minutes of network transfer and processing time to complete the compression of its GOP (i.e. 1/2 second of video). At most, therefore, if each set-top box takes the whole 12 minutes to process its 1/2 second of video, 240 set-top boxes working in parallel with each DDR would be required, or 1440 total set-top boxes.

If the set-top boxes do not need all 12 minutes to complete their respective subtasks, the number of boxes required drops proportionately. For example, if each set-top box completes its MPEG processing in 3 minutes, then only 360 set-top boxes are required. However, the outbound data channel (CPF-to-network) would then have to support a rate of 40 MB per second per outbound channel instead of 10 MB per second. The inbound rate would have to be 400 KB per second per inbound channel (network-to-CPF) instead of 100 KB per second.

If each set-box took more than 12 minutes to process a set of video frames, the total number of set-boxes would have to be increased, but the outbound and inbound data channel rates would also decrease proportionately.

Using the set-top boxes 12 in the massively parallel distributed computing system 10 to process input video image data (static or actual dynamic broadcast), an MPEG compression ratio of at least 100:1 can be achieved in real-time. In the preferred embodiment, the associated delay is always less than thirty minutes, irrespective of the duration of the input video data.

In state two (30), the set-top boxes may decompress video data that they earlier helped to compress. For example, if a subscriber requests an on-demand video service, MPEG compressed video data is retrieved from archival storage 54 and sent to the subscriber's set-top box. As noted, each set-top box contains MPEG decompression hardware 22 which is used to quickly and efficiently decompress MPEG compressed video before the video is viewed. The MPEG decompression is done in real-time as the MPEG compressed video frames arrive from the computer system 10.

Naturally, the data stored in archival storage 54 can be copied and used to provide compressed programming data for distribution on cable systems other than the one on which the data was compressed.

In another form of the invention, JPEG compression (ISO/IEC standard 10918) is advantageously used to reduce the system's data transfer and/or storage requirements. JPEG is a compression technique well known in the prior art for compressing individual still image or video frames. In systems according to this embodiment, dedicated JPEG processors are employed--either before the source data is stored on DDRs 48, or in DDR output bus 62--to JPEG compress each frame of video before transmission to the set-top boxes. On receipt, each set-top box decompresses the JPEG-encoded data before performing the MPEG compression. The JPEG compression/decompression process helps eliminate high frequency noise common in video sequences, and improves the MPEG compression ratio.

If JPEG compression is used before DDRs 48, the storage capacity of the DDRs is reduced by the JPEG compression ratio (typically 2:1 to 10:1), and the bandwidth off the disks is reduced by a like factor, thereby reducing substantially the computer processing required. While JPEG compression ratios of up to 100:1 or more are possible, very high compression ratios should be avoided if broadcast quality output is to be maintained.

For expository convenience, the foregoing discussion has referred only to video signals. It should be understood that this is meant to include audio, which is typically associated with video signals, at least in the field of entertainment programming. The ISO/IEC MPEG II standard contemplates compression of audio with the video, and compression effected by the illustrated embodiment includes compression of the associated audio track.

While the preferred embodiment has been illustrated by reference to the computationally intensive task of broadcast quality MPEG compression, it will be recognized that the disclosed systems and techniques are equally applicable to any other computationally intensive task. Preferably, such alternative tasks are, by their nature, well suited for partitioning into a multitude of parallel tasks and, desirably, do not require extensive local storage at each processing node. Protein or DNA homology searching (fragment matching) is but one such application. A variety of other tasks well suited for execution on such a massively parallel processor will be apparent to artisans in various fields.

Having illustrated and described the principles of the invention in a preferred embodiment, it should be apparent to those skilled in the art that the embodiment can be modified in arrangement and detail without departing with such principles. For example, while the invention has been illustrated with reference to an embodiment in which the processing nodes communicate only with the central processing facility 50, and not with each other, in other embodiments inter-node communications can advantageously be employed. Similarly, while the invention has been illustrated with reference to an embodiment in which data is transmitted over the network 14 in digital form, it will be recognized that the same architecture can be used to pass and process analog data. In some embodiments, the network can carry both analog and digital data, with the analog data being frequency multiplexed on top of the digital data, or relayed in another non-interfering fashion.

Still further, while various elements of the illustrated embodiment have been described as being implemented in hardware, in other embodiments software implementations may alternatively be used, and vice versa.

In view of the wide variety of embodiments to which the principles of my invention can be applied, it should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of my invention. Rather, I claim as my invention all such embodiments as come within the scope and spirit of the following claims and equivalents thereto. 

I claim:
 1. In a parallel processor including a control station coupled to a plurality of processing nodes, the control station serving to partition a processing task and distribute partitioned subtasks thereof to the processing nodes for processing, the control station further serving to collect processed results from said nodes and use said results to produce a final output to said processing task, an improvement comprising an asymmetrical network coupling each of the processing nodes to the control station, said network providing a first bandwidth link for coupling data from the control station to the processing nodes, and a second bandwidth link for coupling data from the processing nodes back to the control station, said first bandwidth being at least twice the second bandwidth, wherein the network resources are efficiently allocated to handle processing tasks that can be partitioned into subtasks which have more input data than output data.
 2. The parallel processor of claim 1 in which different of said processing nodes are available at different times to receive partitioned subtasks of said processing task, due to occasional use of said nodes for other processing jobs, said other processing jobs being unrelated to said processing task, the scheduling of said other processing jobs not being under the control of the control station.
 3. The parallel processor of claim 1 in which the network includes a wireless link for coupling data from the control station to the processing nodes.
 4. The parallel processor of claim 1 in which the network includes a terrestrial link for coupling data from the processing nodes back to the control station.
 5. The parallel processor of claim 1 in which the network includes nodes located in a plurality of different states.
 6. The parallel processor of claim 1 in which a plurality of said nodes are set-top boxes usable in conjunction with cable television receivers.
 7. The parallel processor of claim 1 in which said network further includes means for providing entertainment programming to nodes that are not provided subtasks of said processing task.
 8. The parallel processor of claim 1 in which the same physical medium is used both to couple data from the control station to the processing nodes, and to couple data from the processing nodes back to the control station.
 9. The parallel processor of claim 1 in which the plurality of processing nodes comprise plural cable television receivers, each television receiver including a processing unit, each television receiver having a first state in which the processing unit therein is performing a subscriber function for display of video to a user, and a second state in which said processing unit is not performing a subscriber function, the receiver including means for changing between the first and second states; the parallel processor further including a plurality of distribution switches, each coupled to a plurality of said cable television receivers, and a central facility including mass storage means and a control processor, each of the distribution switches being coupled to said central facility, wherein a network comprising the distribution switches and the coupling between the central facility and the television receivers is asymmetrical in transmission speed, allowing data transmission to the television receivers from the central facility at a rate at least ten times that of data transmission to the central facility from the television receivers.
 10. In a parallel processor including a control station coupled to a plurality of remote processing nodes, the control station serving to partition a processing task and distribute partitioned subtasks thereof to the processing nodes for processing, the control station further serving to collect processed results from said nodes and use said results to produce a final output to said processing task, an improvement wherein each of the processing nodes is switchable between primary and secondary modes of operation, in the secondary mode the node being available to process a subtask distributed thereto by the control station, in the primary mode the node being preempted from subtask processing by local usage of the node for a task unrelated to said processing task, said local usage not being invoked by the control station but being scheduled by a human user, and wherein each node includes means for automatically indicating to the control station if it is in said secondary mode of operation.
 11. The parallel processor of claim 10 in which a plurality of said processing nodes comprise television set-top boxes.
 12. The parallel processor of claim 10 in which the plurality of remote processing nodes comprise plural cable television receivers, each television receiver including a processing unit, in the primary mode each television receiver performing a subscriber function for display of video to a user, in the secondary mode each television receiver not performing a subscriber function.
 13. The parallel processor of claim 12 in which each receiver includes means for changing between the primary and secondary modes; the parallel processor further including a plurality of distribution switches, each coupled to a plurality of said cable television receivers, and a central facility including mass storage means and a control processor, each of the distribution switches being coupled to said central facility, the control processor including means for discerning whether each of said television receivers is in the secondary mode, the control processor further including means for partitioning a processing task into a plurality of subtasks, means for allocating said subtasks among processing units in said television receivers that have been discerned to be in the secondary mode, means for collecting processed data from said receivers into said mass storage to form a composite set of data results.
 14. The processor of claim 13 in which the control processor further includes recovery means for recovering from a subtask interruption caused by a television receiver being changed from the secondary mode to the primary mode before completing a subtask allocated thereto.
 15. The processor of claim 14 wherein a network comprising the distribution switches and the coupling between the central facility and the television receivers is asymmetrical in transmission speed, allowing data transmission to the television receivers from the central facility at a rate at least ten times that of data transmission to the central facility from the television receivers.
 16. In a parallel processor including a control station coupled to a plurality of processing nodes, the control station serving to partition a processing task and distribute partitioned subtasks thereof to the processing nodes for processing, the control station further serving to collect processed results from said nodes and use said results to produce a final output to said processing task, an improvement comprising means responsive to human intervention at each of said nodes for preempting a processing subtask at said node that is in-process so as to perform a different processing job unrelated to said processing task, and wherein the control system includes means for redundantly distributing a plurality of said subtasks to at least two processing nodes so as to reduce delays in completing the processing task due to such preemptions.
 17. The parallel processor of claim 16 in which the control system includes means for distributing each of said subtasks to at least two processing nodes.
 18. The parallel processor of claim 16 in which a plurality of said processing nodes comprise television set-top boxes.
 19. The processor of claim 16 in which the plurality of processing nodes comprise plural cable television receivers, each television receiver including a processing unit, the processing subtask being use of the television receiver for display of video to a user.
 20. In a multi-processor system employing a central control processor linked to a plurality of distributed processors, the central processor serving to partition a processing task and distribute partitioned subtasks thereof to the distributed processors for processing, the control station further serving to collect processed results from said nodes and to use said results to produce a final output to said processing task, an improvement wherein:each of said distributed processors is linked to the central control processor, but not with others of said distributed processors; the system includes means for providing subtask data as well as control signals from the central control processor to each distributed processor; the central control processor comprises circuitry different than each of the distributed processors; and each link provides a first bandwidth path for coupling data from the central control processor to a distributed processor corresponding thereto, and provides a second bandwidth path for coupling data from said distributed processor back to the central control processor, the first bandwidth being at least twice the second.
 21. The system of claim 20 in which at least one of said distributed processors has primary and secondary operating modes, the processing of said partitioned subtasks being the secondary operating mode and occurring only at times when said distributed processor is not engaged in the primary operating mode.
 22. The system of claim 21 in which each of said plurality of distributed processors has said primary and secondary operating modes.
 23. The multi-processor system of claim 20 in which the plurality of distributed processor comprise plural cable television receivers, each including a processing unit.
 24. A remotely distributed processing system including:a central facility; a plurality of remotely distributed processing units, each unit being switchable between at least first and second modes, in the first mode the unit being unavailable to process a subtask assigned thereto by the central facility, in the second mode the unit being preempted from subtask processing by local usage of said unit, said local usage not being invoked by the central facility, each unit indicating its mode to the central facility; a network coupling each processing unit to the central facility; the central facility including means for partitioning a processing task into a plurality of subtasks, distributing said subtasks to remote processing units indicated as being in the first mode, and receiving subtask results back therefrom; and completion means for completing processing of said processing task notwithstanding interruption of at least one subtask due to switching of a processing unit executing said subtask from the first mode to the second mode before execution of said subtask is completed.
 25. The remotely distributed processing system of claim 24 in which the plurality of remotely distributed processing units comprise plural cable television receivers, each including a processing unit.
 26. The system of claim 24 wherein the central facility further includes a database containing data related to patterns of local usage for said remote processing units.
 27. The system of claim 24 in which the central facility includes a timer for each processing unit allocated a subtask, for sensing subtask interruption by failure of the processing unit to return subtask results data to the central facility within a timed interval.
 28. The system of claim 24 in which the completion means includes means for allocating a given subtask to a plurality of different processing units for redundant processing, wherein interruption of a subtask does not unduly interrupt processing of said processing task.
 29. The system of claim 28 which further includes means for comparing subtask results returned from at least two of said different processing units.
 30. The system of claim 28 in which the central facility further includes a database containing data useful in determining how many different processing units to use in redundantly processing a given subtask, in view of patterns of local usage of said processing units.
 31. The system of claim 24 in which the completion means includes reallocation means at the central facility for reallocating an interrupted subtask to a different, substitute processing unit.
 32. The system of claim 31 in which said reallocation means includes means for providing restoration data from the central facility to the substitute processing unit to allow it to resume processing of the interrupted subtask from an intermediate state, rather than starting over.
 33. The system of claim 31 in which the reallocation means includes means for reinitializing the substitute processing unit to start the interrupted subtask from the beginning thereof.
 34. A remotely distributed processing system including a central facility and a plurality of remotely distributed processing units, each of the processing units being switchable between first and second modes, in the first mode the processing unit being available to process a subtask assigned thereto by the central facility, in the second mode the processing unit being preempted from subtask processing by local usage of the unit, said local usage not being invoked by the central facility, the central facility including means for partitioning a processing task into a plurality of subtasks, distributing said subtasks to processing units in the first mode, and receiving subtask results back therefrom, the central facility including a database containing data relating to historical patterns of local usage for said processing units, wherein said subtasks can be allocated among said processing units in accordance with said historical usage patterns.
 35. The remotely distributed processing system of claim 34 in which the plurality of remotely distributed processing units comprise plural cable television receivers, each including a processing unit, said local usage of a remotely distributed processing unit comprising use of the cable television receiver to view video.
 36. The system of claim 34 in which the central facility includes means for distributing a given subtask to a plurality of different processing units for redundant processing, wherein interruption of a subtask by switching of a processing unit to the second mode prior to execution of a subtask distributed thereto does not unduly interrupt processing of the processing task.
 37. The system of claim 36 which includes means for determining the number of redundant processing units in accordance with historical usage data stored in the database. 